1. Field of the Invention
The present invention relates to polymers based on styrene derivatives and vinylcyclohexane derivatives comprising hydroxyl groups and acid-labile 1-alkoxyethyl side groups, to radiation-sensitive compositions comprising these polymers, to processes for the production of images using these compositions, and the use of compositions as positive photoresists.
2. Brief Description of the Relevant Art
The production of highly integrated circuits currently requires the ability for structures having a width of less than 0.35 .mu.m to be transferred imagewise to the substrate. For this reason, there is increasing interest in photoresists which work at wavelengths in the deep UV region (DUV region, deep ultra violet: 200 to 300 nm). Novolaks, which are frequently used in conventional positive photoresists, absorb so strongly in this region that uniform exposure over the usual layer thickness of approximately 1.0 .mu.m is impossible. In order to ensure the requisite optical transparency, poly(p-hydroxystyrene) or derivatives thereof are generally used as binder resins for photoresists working in the DUV region.
Positive photoresists of increased radiation sensitivity comprise, in general, binders obtained from alkali-soluble resins by blocking the alkali-solubilizing groups (usually hydroxyl groups) with acid-labile protecting groups, thereby rendering the binder resin substantially insoluble in alkali. Exposure then initiates a primary reaction of an acid photogenerator which absorbs at the appropriate wavelength, forming a strong acid which, in the subsequent secondary reaction, results in elimination of the protecting groups and thus in re-formation of the solubilizing groups. In the elimination reaction, the acid is liberated again and catalyses further deprotection cycles. This principle is commonly referred to as "chemical reinforcement".
An example which may be mentioned of such systems is poly(p-hydroxystyrene) whose phenol groups are protected by, for example, tert-butyloxycarbonyloxy (tBoc) or tetrahydropyranyl (THP) groups. Photoresists comprising such binder resins are known (see for example: M. J. Bowden and S. R. Turner (eds.) "Electronic and Photonic Application of Polymers", ACS Series 218, Washington 1988; and N. Hayashi et al., Polymer 33, 1583-1588 (1992)), but have disadvantages concerning adhesion to silicon.
A further disadvantage of these chemically reinforced resists is the sensitivity in terms of the process interval between exposure and the subsequent step of drying to completion. Extended intervals (post exposure delay), which become critical after only a few minutes in sensitive systems, result, in the case of chemically reinforced positive resists, in considerable surface inhibition, leading to the formation of T-shaped denatured profiles (S. A. MacDonald et al., SPIE Vol. 1466, Advances in Resist Technology and Processing VIII (1991) 2-7).
Furthermore, the modification of the latent image after exposure and prior to developing constitutes a problem. This gives rise to resist structures having dimensions which are smaller than the dimensions of the imaging mask used (M. Padmanaban et al., J. Photopolym. Sci. Technol. 7, 461-472 (1994)).
These known binder resins have a high protecting-group content in order to ensure a maximum difference in solubility between the irradiated and nonirradiated areas. Owing to the radiation-induced elimination reaction, in the case of these resins with a high protecting-group content a considerable proportion of the resist material is removed, which can lead to an undesirably high degree of removal of the thickness of the photoresist film in the exposed areas.
Furthermore, in order to achieve a good lithographic contrast it is necessary for the majority, i.e. more than 90%, of the protecting groups to be eliminated during the secondary reaction (O. Nalamasu et al., SPIE Vol. 1262, Advances in Resist Technology and Processing VII (1990) 32-48).
For this reason there is increased use of partially protected polymers having a protecting-group content of not more than 50% as binders in photoresist compositions. EP-A 520,642, for example, describes high-resolution DUV photoresists which comprise p-hydroxystyrene polymers having up to 50% of acid-labile acetal and ketal protecting groups in combination with customary acid photogenerators.
A disadvantage of partially protected polymers having free phenolic hydroxyl groups is the relatively low thermal stability, which can be attributed to partial deprotection by the catalytic influence (autocatalysis) of the weakly acid phenolic hydroxyl groups (H. Ito, "Solid-State Thermolysis of Poly(p-t-butoxycarbonyloxystyrene Catalyzed by Polymeric Phenol: Effect of Phase Separation", J. Polym. Sci., Part A, 24, 2971-2980 (1986)). The processing scope and the shelf life of such photoresists are severely limited by this, especially when the acid lability of the protecting groups used is high. An undesirably high solubility in the unexposed area-may be the result.
On the other hand, an increase in protecting-group content is generally accompanied by a decrease in the glass transition temperature Tg of the polymers and thus in the flow resistance as well (dimensional stability of the relief structures produced).
U.S. Pat. No. 5,258,257 discloses photoresists which comprise an acid photogenerator and a partially hydrogenated poly(hydroxystyrene) in which not more than 50% of the hydroxyl groups have been blocked by acid-cleavable protecting groups. These photoresists are said to have a high transparency in the DUV region and to give high-resolution relief structures with near vertical edges; the post exposure delay time stability, however, is inadequate.